This invention relates to thermal radiation detection devices and, in particular, to an improved apparatus for detecting infrared radiation including a protective film such as vanadium trioxide for protecting a detector array against excessive amounts of thermal radiation.
Infrared photo detectors are operated at low temperatures in order to obtain improved detector performance The detectors are typically mounted in a double flask of the Dewar type such as disclosed in U.S. Pat. No. 4,206,354 to Small, Jr., the teachings of which are incorporated herein by reference. The infrared detectors are typically mounted in thermal contact with the top surface of an inner flask, for example, which is cryogenically cooled in order to allow the detector array to operate at cryogenic temperatures. Detector arrays are typically comprised of material such as HgCdTe, and other radiation sensitive materials. In a typical detection system, system optics focus radiation through a transmitting window which may be typically germanium, for example, and onto the detector array. In prior art systems, no provision is made for protecting the detector array from excessively high amounts of thermal radiation, as may be produced by a coherent, collimated light source.
Vanadium trioxide (V.sub.2 O.sub.3) is a material that is transparent in the long wave infrared (LWIR) spectrum until it reaches a transition temperature of about 150.degree. K. At such transition temperatures, it becomes reflective. The present invention takes advantage of this property by placing a film of V.sub.2 0.sub.3 on a transmissive element in an LWIR detector system. The film is located very near a detector array in an infrared detection apparatus. In accordance with the present invention, such a V.sub.2 O.sub.3 coated transmissive element or substrate provides hardening against interference from intense light sources and damage from such light sources. Further, the present invention provides for locating the protective film proximate to the detectors near the focus of the system optics so as to localize the area so protected and allow the other detectors to receive more normal thermal radiation signals. As an intense amount of radiation is presented to the detector, the V.sub.2 O.sub.3 film is heated by the radiation above its threshold temperature and becomes reflective, thereby reflecting the intense thermal radiation away from the detector array. Because the V.sub.2 O.sub.3 film is nearly in focus, only the exact area being exposed to the intense thermal radiation is switched. The rest of the detector array is still operable to receive radiation coming from other sources.